1. Field of the Invention
The present invention relates to a method for producing a functional nanocarbon and hydrogen by direct decomposition of a lower hydrocarbon such as methane in the presence of a catalyst.
2. Description of the Related Art
It has heretofore been known that a reaction in the presence of a catalyst causes the production of carbon (see, e.g., JP-A-10-182121). JP-A-10-182121 proposes that the reaction of carbon dioxide with hydrogen causes the production of carbon. Further, a method has been recently proposed which comprises subjecting a lower hydrocarbon containing methane to direct decomposition in the presence of a catalyst to produce a functional nanocarbon and hydrogen. Moreover, JP-A-2004-269398 proposes a process for the production of hydrogen and an aromatic hydrocarbon from a lower hydrocarbon wherein hydrogen is incorporated in the raw material of hydrocarbon in an amount of 20 vol % or less so that carbon deposited in the pores of zeolite or on the surface of zeolite is converted back to methane which is then removed.
In order to enhance the percent one pass conversion in the above related arts, it is necessary that the reaction temperature be raised as much as possible because the direct decomposition reaction of a lower hydrocarbon into carbon and hydrogen is endothermic. However, when the reaction temperature is raised, the decomposition reaction rate of the lower hydrocarbon is raised, causing the production of a large amount of various solid carbon materials on the surface of the catalyst. These solid carbon materials include functional nanocarbon materials which are desired products (e.g., carbon nanofiber, carbon nanotube, onion-like carbon), precursor of carbon nanofiber, and secondarily produced amorphous carbon. The turbostratic carbon constituting carbon nanofiber or onion-like carbon can be definitely distinguished from amorphous carbon by X-ray diffractometry or Raman spectroscopy. When the functional nanocarbon precursor or amorphous carbon occurs on the surface of the catalyst in excess amount, they react with the catalyst metal to produce an inactive metal carbide or physically cover the active sites on the catalyst, preventing the access and adsorption of the lower hydrocarbon to the catalytic active sites and the elimination and diffusion of hydrogen, which is a gaseous product, and hence deteriorating the rate of decomposition reaction of the lower hydrocarbon to functional nanocarbon and hydrogen. As a result, conversion (percent conversion) of the lower hydrocarbon decreases with time, reducing the time interval between catalyst replacements to disadvantage. The direct decomposition reaction of a lower hydrocarbon is greatly different from ordinary gas-solid catalytic reaction in that one of the products is a solid carbon material that remains on the surface of the catalyst. Therefore, it has been heretofore considered that the drop of the conversion of lower hydrocarbon with time is unavoidable.
In the method for producing a functional nanocarbon and hydrogen by direct decomposition of a lower hydrocarbon in the presence of a catalyst, it has been heretofore practiced to use as a raw material a high purity lower hydrocarbon which has been freed of components inhibiting the action of the catalyst as much as possible. Therefore, an apparatus for purifying the lower hydrocarbon is needed. The fixed cost and operating cost required for this purifying apparatus add to the production cost. For example, when methane obtained by the purification of a biogas is used as lower hydrocarbon, carbon dioxide that accounts for the biogas in a proportion of from 30% to 35% must be separated. In a hollow fiber membrane separating method, carbon dioxide normally remains in methane in a proportion of about several percents. In order to further purify the biogas, so-called PSA method must be employed, raising an economical problem that the burden of fixed cost and operating cost occurs separately.